Electrical machines generally comprise a rotor structure and a stator structure. In case of permanent magnet excited generators (PMG) permanent magnets (PM) are generally comprised in the rotor (although they could also be arranged alternatively in the stator structure), whereas winding elements (e.g. coils) are usually included in the stator (although they could alternatively be arranged in the rotor structure).
The insertion of a rotor of a PM generator carrying already magnetized magnet elements into a pre-assembled stator, i.e. a stator structure plus winding elements and stator core can be complicated due to the high attraction forces generated by the magnets between the stator core and the magnets (i.e. between stator and rotor). These magnetic forces pull the rotor towards the stator and lead to an inherently unstable situation with respect to the radial centering, i.e. a small eccentricity of the rotor may lead to high magnetic forces trying to increase the eccentricity. As a result, not only expensive and voluminous tooling is generally required for this operation, but the operation can also entail a significant health and safety risk.
In alternative methods, in order to reduce these attraction forces, it has been proposed to first assemble a rotor structure without magnets with a stator structure carrying windings. Because no magnetic elements are provided, this assembly can be relatively easy.
Then, subsequently, permanent magnet modules can be mounted onto the rotor structure. Such permanent magnet modules may advantageously be slid into a slot provided on a rotor rim.
A permanent magnet module may be defined as a unit having a plurality of permanent magnets, such that the plurality of magnets can be mounted and unmounted together. Such a module may have a module base with a shape suitable for housing a plurality of permanent magnets. The magnets may be fixed to the base in a variety of manners. The base may be configured to be fixed to a rotor rim in such a way that the plurality of magnets are fixed together to the rotor rim through the module base. The use of permanent magnet modules can facilitate the manufacturing of a generator rotor.
Even though this method of assembly may generally be easier than the previous one, also in this method the magnetic attraction when inserting the permanent magnet modules may cause problems and more so when a number of modules have already been mounted. Again heavy and costly guiding systems and tools are generally required. Furthermore, the operation may be time consuming due to the high number of modules to be inserted.
The present disclosure aims at improving the prior art methods of assembling an electrical machine such as e.g. a PMG.